Illuminated signs based on light-emitting diodes instead of incandescent lamps are being used increasingly in many areas, especially for signaling. Light-emitting diodes are comparatively cost-effective, have a long life and produce a strong light. U.S. Pat. No. 5,939,839 discloses a circuit for protecting the light-emitting diodes of a light signal against overvoltages. European Application No 0 293 921 relates to a circuit which protects light-emitting diodes against overheating. However, it is difficult to use light-emitting diodes where incandescent lamps are intended to be replaced by LED illuminated signs without any changes to the drive. This is particularly true of light signal circuits for railroads, in which correct operation is generally monitored by current measurement which is safe from the signaling point of view. In order to allow this monitoring still to be used without any changes, the current/voltage characteristic of the LED illuminated sign must correspond approximately to that of an incandescent lamp.
A further special feature is signaling outside tunnels or environmental conditions with approximately constant light characteristics. In this case, the circuitry must provide a reduction in the light power for night time operation, in comparison to that for daytime operation. The light sensitivity of the human eye during daytime differs by a factor of approximately 1000 from the light sensitivity at night. If the light power is not reduced at night, dazzling is therefore possible even in the case of lights which are barely perceptible in the daytime. However, it is essential to avoid dazzling, particularly for road or rail traffic, since there is a risk of other signals effectively possibly being overlooked due to the excess radiation. In the case of light signals based on incandescent lamps for railroad purposes, the brightness is controlled between daytime and night by the control mechanism using the supply voltage or the supply current. Since the light power of an incandescent lamp depends exponentially on the supply voltage or the supply current, a small change in the supply current or the supply voltage leads to a major change in the light power. This means that the supply current or the supply voltage need be reduced only to about ⅔ of the initial value in order to reduce the light power, for example, to 20% of the initial light intensity. In order to achieve a similarly advantageous characteristic profile for light-emitting diodes, a proposal has been made, according to DE 198 46 753 A1, for a drive circuit to be connected in parallel with each light-emitting diode. This has the disadvantage that the achievable difference between the light power in the daytime and the night is relatively small. Furthermore, component tolerances which lead to different forward voltages on the light-emitting diodes, transistors and other components, as well as the influence of temperature on the forward voltages, are not compensated for.